Silver halide photothermographic imaging materials, often referred to as `dry silver` compositions because no liquid development is necessary to produce the final image, have been known in the art for many years. These imaging materials basically comprise a light insensitive, reducible silver source, a light sensitive material which generates silver when irradiated, and a reducing agent for the silver source. The light sensitive material is generally photographic silver halide which must be in catalytic proximity to the light insensitive silver source. Catalytic proximity is an intimate physical association of these two materials so that when silver specks or nuclei are generated by the irradiation or light exposure of the photographic silver halide, those nuclei are able to catalyze the reduction of the silver source by the reducing agent. It has been long understood that silver is a catalyst for the reduction of silver ions and the silver-generating light sensitive silver halide catalyst progenitor may be placed into catalytic proximity with the silver source in a number of different fashions, such as partial metathesis of the silver source with a halogen-containing source (e.g., U.S. Pat. No. 3,457,075), coprecipitation of the silver halide and silver source material (e.g., U.S. Pat. No. 3,839,049), and any other method which intimately associates the silver halide and the silver source.
The silver source used in this area of technology is a material which contains silver ions. The earliest and still preferred source comprises silver salts of long chain carboxylic acids, usually of from 10 to 30 carbon atoms. The silver salt of behenic acid or mixtures of acids of like molecular weight have been primarily used. Salts of other organic acids or other organic materials such as silver imidazolates have been proposed, and U.S. Pat. No. 4,260,677 discloses the use of complexes of inorganic or organic silver salts as image source materials.
In both photographic and photothermographic emulsions, exposure of the silver halide to light produces small clusters of silver atoms. The imagewise distribution of these clusters is known in the art as a latent image. This latent image generally is not visible by ordinary means and the light sensitive article must be further processed in order to produce a visual image. The visual image is produced by the catalytic reduction of silver ions which are in catalytic proximity to the specks of the latent image.
As the visible image is produced entirely by silver, one cannot readily decrease the amount of silver in the emulsion without reducing the available maximum image density. Reduction of the amount of silver is desirable in order to reduce the cost of raw materials used in the emulsion.
One traditional way of attempting to increase the image density of photographic and photothermographic emulsions without increasing or while decreasing the amount of silver in the emulsion layer is by the addition of dye forming materials into the emulsion. In this way a dye enhanced silver image can be produced, as for example in U.S. Pat. Nos. 3,531,286, 4,187,108, 4,426,441, 4,374,921 and 4,460,681.
It has been described in the patent literature to transfer a dye image formed in a photothermographic system by means of a transfer solvent as is disclosed, for example, in U.S. Pat. Nos. 3,985,565, 4,021,240, 4,022,617, 4,430,415, 4,463,079, 4,455,363, 4,499,172, 4,499,180, and 4,503,137.
Japanese Kokai No. 59-5239 discloses a photothermographic contact diffusion system wherein a chemical reaction occurs in an image receiving layer between a diffused leuco dye and an acidic color developing agent.